The sleep problems of older men and women often have timing difficulties as a significant part of their etiology. A misalignment of the circadian system ("biological clock") of older people by only a few hours can lead to significant sleep impairment. Thus, an examination of the effects of shifts in the timing of sleep may lead to therapeutic strategies which could alleviate the sleep disorders of older people without resorting to hypnotic medications. In our previous work we have studied the tolerance of elderly people to phase shifts of 6h. We now propose to investigate rather smaller (2h) phase shifts. We shall compare three groups: older men, older women, and young controls, in their response to both phase delays and phase advances, with all subjects doing control, advance and delay conditions. We also plan to investigate the relationship between circadian rhythms and sleep by indexing the bedtime (control or shifted) to its phase difference from the phase of the endogenous circadian pacemaker. We shall use two well accepted circadian phase markers: 1) the time of minimum [Tmin] of the rectal temperature rhythm as fitted by 24h and 12h sinusoids, and 2) the timing of Dim Light Melatonin Onset (DLMO) as measured by half-hourly evening salivary samples. Each subject will experience three conditions in three separate 120h runs. In each run, after two baseline days and nights (48h) at the subject's habitual routine, subjects will live for 3 days and nights under ad-lib sleep length (time in bed) conditions, but with the timing of bedtime specified. First, in the control condition, the bedtime will always be at habitual bedtime, so that baseline levels of sleep and daytime performance can be obtained and the phase difference between Tmin and bedtime, and between DLMO and bedtime later calculated from the entire 5-day run. In the advance condition the bedtime on nights 3-5 will be two hours earlier than habitual bedtime. In the delay condition bedtime on nights 3-5 will be two hours later than habitual bedtime. In all cases, sleeps will be polysomnographically recorded, rectal temperatures monitored continuously, and daytime alertness, mood and performance assessed using our usual battery of tests. We propose to study 10 older (70y+) women, 10 older (70y+) men and 10 young (20y - 35y) controls (5m, 5f), with order of presentation of the advance and delay conditions counterbalanced. Hypotheses will be tested relating both to the effect of phase shift on nocturnal sleep and daytime alertness and performance variables, and also to the effect on these variables of individual daily variations in observed circadian phase angle between Tmin and bedtime, and between DLMO and bedtime.